fix: update runner golang image from 1.25 to 1.26

kmsvc-manage, kmsvc-cli require Go 1.26.0 in go.mod but runner was
using golang:1.25-bookworm container. Update to golang:1.26-bookworm
to match project requirements.

k8s/infra/forgejo-runner/values.yaml

@@ -9,7 +9,7 @@ runner:
   # instance carries the golang toolchain and its own dind sidecar builds and
   # pushes that repo's images too -- there is no separate generic runner
   # anymore.
-  labels: "golang:docker://golang:1.25-bookworm"
+  labels: "golang:docker://golang:1.26-bookworm"
   # In-cluster Service (:3000) — direct, avoids the ingress/public-hostname hop
   # (the public URL is :443 which forgejo doesn't serve; runner got i/o timeout).
   forgejoUrl: http://forgejo-gitea-http.cicd.svc.cluster.local:3000